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vein

The Columbia Encyclopedia, Sixth Edition

vein blood vessel that returns blood to the heart . Except for the pulmonary vein, which carries oxygenated blood from the lungs to the heart, veins carry deoxygenated blood. The oxygen-depleted blood passes from the capillaries to the venules (small veins). The venules feed into larger veins, which eventually merge into the superior and inferior vena cavae, large vessels that consolidate the blood flow from the head, neck, and arms and from the trunk and legs, respectively (see also circulatory system ). The vena cavae direct the blood back into the heart. The walls of a vein are formed of three layers like the walls of an artery. However, these layers are thinner and less muscular and collapse when empty. With such notable exceptions as the portal system, most veins contain valves, formed by pouches in their inner coats, that keep the blood from flowing backward. Valves are most numerous in the veins of the extremities, and are absent in the smallest veins. Veins are subject to inflammation, dilatation or enlargement (as in a varicose vein ), rupture, and blockage by blood clots ( thrombosis ).


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The Columbia Encyclopedia, Sixth Edition. Copyright 2008 Columbia University Press

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Related entries from encyclopedias, dictionaries, and thesauruses

Portal Vein Bypass
Portal vein bypass Definition Portal vein bypass surgery diverts blood from the portal vein into another vein. It is performed when pressure in the portal vein is so high that it causes internal bleeding from blood vessels in the esophagus. Purpose... Read more
Renal Vein Thrombosis
Renal vein thrombosis Definition Renal vein thrombosis develops when a blood clot forms in the renal vein, which is the blood vessel that carries blood from the kidneys back to the heart. The disorder is not common. Description Normally, kidneys... Read more
Vein Ligation and Stripping
Vein ligation and stripping Definition Vein ligation and stripping is a surgical approach to the treatment...phlebectomy. Ligation refers to the surgical tying off of a large vein in the leg called the greater saphenous vein, while stripping refers... Read more
vein
vein / vān / • n. 1. any of the tubes forming part...other abstract resource: he managed to tap into the thick vein of discontent to his own advantage. 2. [in sing. ] a distinctive...or tendency: he closes his article in a somewhat humorous vein. DERIVATIVES: vein·less adj. ... Read more
varicose vein
or varix Twisted vein distended with blood. Varix also covers arteries...skin. Causes include hereditary valve and vein wall weakness and internal or external pressure...ulcerate and bleed. hemorrhoid . varicose vein varicose vein varicose vein Read more

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